55 Dating Old Radios By Their Tube Complement: p3 Older radios
Description
This article is from the Antique Radios And Phonographs FAQ, by Hank van Cleef vancleef@netcom with numerous contributions by others.
55 Dating Old Radios By Their Tube Complement: p3 Older radios
Development of a button seal that could be used with glass envelopes
allowed manufacture of metal-based "loctal" tubes. These entered
production in 1939. At the same time, a cylindrical bulb for glass
tubes also entered production, allowing closer spacing between tubes.
Experimental FM became a commercial broadcast enterprise in 1940. The
original FM band began at 42 megacycles, and production of home
entertainment receivers to receive that band began in 1941. The band
originally overlapped the experimental television band (later channel 1,
48-54 megacycles). The FM band was reallocated to 88-108 megacycles in
the spring of 1945, thus a set with 88-108 capability is postwar.
Another "strictly postwar" feature is the 7-pin miniature tube. The
9-pin miniature followed around 1949.
A few tubes were "survivors" through the 1928-50 period. The standout
among these is the 80 rectifier, which was still being used in new
production in the mid-1950's. The 5Y3GT which replaced it is nothing but
an octal-based version of the 80. The 2A3 and 45 power triodes, as well
as the less-common 6A3 were all used from the early 1930's until well
into the 1950's. There remains today something of a cult that
believes that these triodes are the only audio power tubes worth
considering. All of these tubes use filament cathodes, and the most
practical circuits for using them required a separate filament winding,
elevated to the 40-60 volts needed to bias these tubes near cutoff.
Beam power tetrodes were introduced as octal tubes, although the 807
(very rarely seen in the home entertainment market) continued to use the
older large 5-pin base. The principal beam power tetrodes were the 6L6,
6V6, and 25/35/50L6. The 6L6 in a push-pull circuit required more
current than a 125 ma. 80 could provide, and presence of a pair of 6L6's
with a bigger rectifier means a "high-end" set. Push-pull 6V6's could
be supplied by an 80 and provide very adequate audio power of good
fidelity to the open-mounted loudspeakers used in virtually all home
entertainment equipment until the mid-1950's. Generally, a push-pull
power output stage, using any pair of triodes, beam tetrodes, or
pentodes, means a quality set with other desireable features, low hum,
and good sensitivity.
The various oscillator-mixer tubes used can affect a radio's ability to
perform, particularly on shortwave bands. Historically, the first such
tube was the 7-pin 2A7/6A7, followed by the octal-based 6A8, all using
the same pentagrid construction and circuit. These operated well on AM
broadcast, but had severe problems dealing with higher frequencies.
While they were commonly used (particularly the 6A8) into the late
forties, they generally give very poor performance on shortwave bands
above 10-15 Mc (40 meters). The 6L7 was developed as a mixer to be
driven by a separate local oscillator to overcome some of the
limitations of the 6A8. The separate-section 6J8 and 6K8 were developed
to provide better high-frequency performance without need for a separate
local oscillator. These tubes can operate well up to about 25 mc. The
loctal versions (7J7, which is the same as a 6J8, and the 7S7, which is
a higher-gain 7J7) would operate over 30 mc. (10 meters.). The final
version was another layout of the 6-grid "pentagrid" design, the 6SA7.
The 6SA7 would operate, with the inner section as an oscillator, up to
about 27 mc. The 6SB7Y octal, 6BE6 7-pin miniature, and 7Q7 loctal all
would operate satisfactorily up the commercial FM frequencies. A common
method for getting better high-frequency performance was to use a
separate local oscillator with a 6L7, 6SA7, or 6BE6. Glow-discharge
voltage regulator tubes were commonly used in high-end communications
designs to regulate B+ to the local oscillator, giving improved
stability to the circuit. For serious shortwave listening, you should
avoid a set with a 6A7 or 6A8, and consider one with a separate local
oscillator (typically a 6C5, 6J5, or 6C4) and a voltage regulator tube.
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